56 



facturer to use much poorer and therefore cheaper grades of 

 coal than could otherwise be utilized. 



CLiNKER GRINDING. GYPSUM. 



Clinker grinding. The power and machinery required for 

 pulverizing the clinker at a Portland cement plant using the dry 

 process of manufacture is very closely the same as that required 

 for pulverizing the raw materials for the same output. This 

 may seem, at first sight, improbable, for Portland cement clinker 

 is much hardier to grind than any possible combination of raw 

 materials ; but it must be remembered that for every barrel of 

 cement produced about 600 pounds of raw materials must be 

 pulverized, while only a scant 400 pounds of clinker will be 

 treated, and that the large crushers required for some raw ma- 

 terials can be dispensed with in crushing clinker. With this ex- 

 ception, the raw material side 'and the clinker side of a dry-pro- 

 cess Portland cement plant are usually almost or exactly dupli- 

 cates. 



The difficulty, and in consequence the expense, of grinding 

 clinker will dlepend in large part on the chemical composition of 

 the clinker and on the; temperature at which it has been burned. 

 The difficulty of grinding, for example, increases with the per- 

 centage of lime carried by the clinker ; and a clinker containing 

 64 per cent, of lime will be very noticeably more resistant to 

 pulverizing than one carrying 62 per cent, of lime. So fair as 

 regards burning, it may be said in general, that the more thor- 

 oughly burned the clinker the more difficult it will be to grind, 

 assuming that its chemical composition remains the same. 



The tendency among engineers at present is to demand more 

 finely ground cement. While this demand is doubtless justified 

 by the results oi comparative tests of finely and coarsely ground 

 cements, it must be borne in mind that any increase in fineness 

 of grinding means a decrease in the product per hour of the 

 grinding mills employed, and a consequent increase in the cost 

 of cement. At some point in the process, therefore, the gain in 

 strength due to fineness of grinding will be counterbalanced by 

 the increased cost of manufacturing the more finely ground pro- 

 duct. 



The increase in the required fineness has been gradual but 

 steady during recent years. Most specifications now require at 



